1. Field of the Invention
The present invention relates to a surface acoustic wave filter used in a filter for a pager or the like and, more particularly, to a surface acoustic wave filter capable of preventing electrostatic breakdown between filter patterns formed on a substrate surface.
2. Description of the Prior Art
In a surface acoustic wave filter, electrode portions, each obtained by interdigitally forming a pair of comb electrodes on a piezoelectric substrate to oppose each other at a predetermined distance, are arranged on the input and output sides, respectively. In order to use one and the other of the two arranged electrode portions as input and output side electrode portions, respectively, an input terminal is connected to one comb electrode of the input side electrode portion, an output terminal is connected to one comb electrode of the output side electrode portion, and the other comb electrode of each of the input and output side electrode portions is grounded, so that a signal input to the input terminal is output from the output terminal.
In each electrode portion of the surface acoustic wave filter, the adjacent electrode fingers of the pair of comb electrodes have a small gap from each other. When high-voltage static electricity or the like is applied to the electrode portion, a leakage current flows between the comb electrodes.
Such static electricity application occurs not only during normal use but also during the manufacturing process of the surface acoustic wave filter or while the surface acoustic wave is stocked as a final product, when the electrode portion comes into contact with a portion of a surrounding device, and the like, charged with the static electricity.
When a leakage current flows between the comb electrodes, dielectric breakdown occurs between the electrodes, and the product becomes defective.
In order to prevent inconveniences caused by application of the static electricity, each electrode may be coated with an antistatic agent. However, this is not effective for static electricity occurring before the antistatic agent is applied. Furthermore, application of the antistatic agent can sometimes change the characteristics of the surface acoustic wave filter.